1. Field of the Invention
Embodiments relate to a high-frequency high-power terminator, and more particularly, to a terminator having high rated power which operates in a high-frequency range, that is, a radio frequency (RF)/microwave/millimeter-wave range.
2. Description of the Related Art
A high-frequency terminator is used for transceivers of various wireless systems for personal mobile communications and satellite communications to eliminate radio frequency (RF)/microwave/millimeter-wave signals. Recently, there is a trend towards increasing standards of available frequency bands and output power for various wireless systems including fifth generation (5G) wireless communications. Here, RF/microwave/millimeter-wave components using a high-frequency terminator include an isolator, a 90 degree hybrid coupler, a coupled-line directional coupler, ring-hybrid and Lange couplers, magic T, and the like.
As higher power is applied to the RF/microwave/millimeter-wave components, higher rated power is needed for the high-frequency terminator. Here, electromagnetic-wave energy applied to the high-frequency terminator is converted into heat energy, and the heat energy is cooled through a heat sink. Here, when the high-frequency terminator has low rated power, the high-frequency terminator does not withstand the applied power and is burned up.
Thus, the high-frequency terminator is required to maintain input matching in a wide operating frequency range. Further, the high-frequency terminator needs to have high rated power to withstand high input power and not to be destroyed, which is described in detail with reference to FIGS. 1A to 1D. FIGS. 1A to 1D illustrate various forms of conventional high-frequency terminators.
Referring to FIG. 1A, a high-frequency terminator includes an input terminal P1, a transmission line 10, a thin film resistor 11, and a via hole for a ground 12. The ground is realized as a bonding wire or short transmission line in addition to the via hole 12.
FIG. 1B illustrates a schematic circuit diagram of the high-frequency terminator of FIG. 1A. As illustrated in FIG. 1B, the high-frequency terminator is a lumped element including a resistor and a ground. Here, the resistor is generally a film resistor, which includes a thin film resistor and a thick film resistor. Since a thin film resistor has small parasitic inductive and capacitive elements and a small resistance tolerance as compared with a thick film resistor, a thin film resistor is generally used for a high-frequency terminator.
Referring to FIG. 1C, the high-frequency terminator includes an equivalent circuit considering parasitic elements of the high-frequency terminator based on FIG. 1B. Here, the resistor has parasitic capacitance CR and parasitic inductance LR. The ground has parasitic resistance RG, parasitic capacitance CG and parasitic inductance LG. Equation 1 represents characteristic impedance of the conventional high-frequency terminator based on the equivalent circuit of FIG. 1C.
                    [                  Equation          ⁢                                          ⁢          1                ]                                                            Z        =                              {                                          R                                                                            (                                              1                        -                                                                              ω                            2                                                    ⁢                                                      L                            R                                                    ⁢                                                      C                            R                                                                                              )                                        2                                    +                                                            (                                              ω                        ⁢                                                                                                  ⁢                                                  C                          R                                                ⁢                        R                                            )                                        2                                                              +                                                R                  G                                                                                            (                                              1                        -                                                                              ω                            2                                                    ⁢                                                      L                            G                                                    ⁢                                                      C                            G                                                                                              )                                        2                                    +                                                            (                                              ω                        ⁢                                                                                                  ⁢                                                  C                          G                                                ⁢                                                  R                          G                                                                    )                                        2                                                                        }                    +                      j            ⁢                          {                                                                    ω                    ⁢                                                                                  ⁢                                                                  L                        R                                            ⁡                                              (                                                  1                          -                                                                                    ω                              2                                                        ⁢                                                          L                              R                                                        ⁢                                                          C                              R                                                                                -                                                                                                                    C                                R                                                            ⁢                                                              R                                2                                                                                                                    L                              R                                                                                                      )                                                                                                                                                (                                                  1                          -                                                                                    ω                              2                                                        ⁢                                                          L                              R                                                        ⁢                                                          C                              R                                                                                                      )                                            2                                        +                                                                  (                                                  ω                          ⁢                                                                                                          ⁢                                                      C                            R                                                    ⁢                          R                                                )                                            2                                                                      +                                                      ω                    ⁢                                                                                  ⁢                                                                  L                        G                                            ⁡                                              (                                                  1                          -                                                                                    ω                              2                                                        ⁢                                                          L                              G                                                        ⁢                                                          C                              G                                                                                -                                                                                                                    C                                G                                                            ⁢                                                              R                                G                                2                                                                                                                    L                              G                                                                                                      )                                                                                                                                                (                                                  1                          -                                                                                    ω                              2                                                        ⁢                                                          L                              G                                                        ⁢                                                          C                              G                                                                                                      )                                            2                                        +                                                                  (                                                  ω                          ⁢                                                                                                          ⁢                                                      C                            G                                                    ⁢                                                      R                            G                                                                          )                                            2                                                                                  }                                                          (        1        )            
Here, ω is 2π frequency. As indicated by Equation 1, in the impedance of the high-frequency terminator, not only an imaginary part but also a real part changes on a frequency by effects of the parasitic elements.
FIG. 1D illustrates a graph illustrating an example of return loss and characteristic impedance of the conventional high-frequency terminator. Here, the conventional high-frequency terminator is a diamond chip resistor in 0402 size, 50 ohm, manufactured by EMC technology (www.emc-rflabs.com) and has an operating range up to 30 GHz. Here, impedance of an ideal high-frequency terminator needs to have systemic characteristic impedance regardless of a frequency, that is, generally a real part of 50 Ohm and an imaginary part of 0 Ohm. However, as illustrated in FIG. 1D, at a frequency of 5 GHz or greater, the real part and imaginary part of the impedance of the conventional high-frequency terminator start increasing by the effects of the parasitic elements and the return loss becomes worse to 20 dB or less. Such a phenomenon is due to an increase in the effects of the parasitic elements increase with a higher frequency as indicated by Equation 1.
In the end, as a higher frequency is used, the conventional high-frequency terminator is affected by the parasitic elements to deteriorate in reflection coefficient characteristic. Here, the high-frequency terminator employs an open radial stub to improve deterioration in high-frequency characteristics caused by the parasitic elements of the ground via hole, thereby improving the reflection coefficient characteristic based on a particular frequency. However, since the open radial stub operates as grounded capacitance at a high frequency to have a certain bandwidth, the high-frequency terminator has a narrow bandwidth.
                    [                  Equation          ⁢                                          ⁢          2                ]                                                            P        ∝                  KA          h                                                
Equation 2 represents rated power of the high-frequency terminator. Here, P represents power (watt), and K represents thermal conductivity (Watt/mK) of a substrate on which the film resistor is deposited. Further, A represents area of the film resistor, and h represents thickness of the substrate.
As indicated by Equation 2, the high-frequency terminator needs to use a thin substrate with high thermal conductivity in order to increase power consumed in the terminator, that is, rated power. To this end, the high-frequency terminator employs beryllium oxide, aluminum nitride or CVD diamond substrates which are relatively expensive but have high thermal conductivity, instead of an alumina substrate. Also, in order to increase the rated power of the high-frequency terminator, the high-frequency terminator needs to have a large area of the thin film resistor. On the contrary, when the lumped element, such as the thin film resistor, has a one-tenth or smaller size of wavelength of a used frequency, the high-frequency terminator has less effect of the parasitic elements and exhibits original properties. That is, as a higher frequency has a shorter wavelength, the lumped element needs to be small in proportion to wavelength. Therefore, when the size of the thin film resistor is reduced with a higher frequency being used, the rated power of the high-frequency terminator decreases, and there are limitations in increasing the rated power by increasing the area of the thin film resistor.
Thus, since the high-frequency terminator has inferior reflection coefficient characteristic, a narrowband and low rated power, there is a need for a high-frequency high-power terminator with a new structure which achieves broadband matching in RF/microwaves/millimeter waves and has improved rated power.